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1 RSMAS, Division of Marine Biology and Fisheries, University of Miami, Miami, Florida, USA
2 RSMAS, Division of Marine Biology and Fisheries, University of Miami, Miami, Florida, USA; Biology, McMaster University, Hamilton, Ontario, Canada
3 Biological Sciences, Exeter University, Exeter, United Kingdom
4 Kings College, London, United Kingdom
5 RSMAS, Division of Marine Biology and Fisheries, University of Miami, Miami, Florida, USA; Kings College, London, United Kingdom
6 Biology, University of Southern Denmark, Odense, Denmark
* To whom correspondence should be addressed. E-mail: mgrosell{at}rsmas.miami.edu.
Experiments performed on isolated intestinal segments from the marine teleost fish, the European flounder (Platichthys flesus), revealed that the intestinal epithelium is capable of secondary active HCO3- secretion in the order of 0.2-0.3 µmol cm-2 h-1 against an apparent electrochemical gradient. The HCO3- secretion occurs via anion exchange, is dependent on mucosal Cl-, results in very high mucosal HCO3- concentrations and contributes significantly to Cl- and fluid absorption. The present studies were conducted under in vivo like conditions with mucosal saline resembling that of the intestinal fluids in vivo. These conditions result in a transepithelial potential of -16.2 mV (serosal side negative) very different from the -2.2 mV observed under symmetrical conditions. Under these conditions, a significant part of the HCO3- secretion is fueled by endogenous epithelial CO2 hydration mediated by carbonic anhydrase, because acetazolamide (10-4 M) was found to inhibit HCO3- secretion and removal of serosal CO2 was found not to influence HCO3- secretion. Reversal of the epithelial electrochemical gradient for Cl- (removal of serosal Cl-) and elevation of serosal HCO3- resulted in enhanced HCO3- secretion, and enhanced Cl- and fluid absorption. Cl- absorption via an anion exchange system appears to partly drive fluid absorption across the intestine in the absence of net Na+ absorption.
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